X. LIGHTHOUSES, ETC. 543 



Barbier and Fenestre as a specimen of high class M orkrnanship. Each ring is 

 one single piece ; the joints which divide the rings are inclined according to 

 the direction of the refracted ray. The lens is mounted on a pedestal, and 

 revolves around any horizontal axis. 



No. 12. Lenticular panel, dioptric and catadioptric, for flashing lights of 

 the second class, planned by the head engineer, Allard, and constructed by 

 M. Henry-Lepaute, 1876. 



This panel forms part of an apparatus intended for the Pilier lighthouse, 

 situated at the mouth of the Loire, and of which the tower has just been 

 rebuilt. The character given to it in 1829 has been preserved; it is a fixed 

 light varied by flashes every four minutes To produce this character a fixed 

 light apparatus has been adopted, of which two sectors -of ^th horizon, 

 opposed to one another, are replaced by perfect annular lenses ; it revolves 

 at the rate of one turn in eight minutes. In order that the two kinds of 

 lenses maybe adjusted upon the edges, and have a common pinion-jack, the 

 focal distance, which is 0'700 m for the fixed lenses, has been reduced to 

 0'647' n for the annular lenses. The focal lamp has five concentric wicks, 

 instead of four, as usual in lamps of the second class, because the light, being- 

 coloured red in certain directions, it was thought necessary to increase its 

 intensity. 



This panel shows several novel arrangements, some of which are now 

 applied for the first time. 



1st. In the central or dioptric parts of the section, the joints that divide 

 the elements, and therefore the lower sides of these elements, instead of being 

 horizontal, are inclined according to the direction of the refracted ray. This 

 system has several advantages : it does away with a triangular part of glass 

 which is useless, and thus lessens the weight of the apparatus; it reduces in a 

 large proportion the loss of light caused by horizontal joints ; it makes less 

 acute, and consequently less fragile, the outer angles of the elements, and, 

 besides, it diminishes their projection, thus enabling the dioptric lens to 

 have a greater height. 



2nd. The central lens (or dioptric) comprehends a vertical angle of 76 

 degrees, whereas, in the old sections, this angle was of about 60 degrees only ; 

 its elevation is thus increased from 0'85 m to l-10 m . This advantage is thus 

 obtained, that the luminous rays meet the last dioptric element at the same 

 angle as the first catadioptric ring, and suffer no more loss of reflection upon 

 the one than upon the other. 



3rd. The section commonly used in apparatus of the second class had been 

 calculated for a three-wick lamp burner of 0'074 m diameter. With a five-wick 

 burner of 0'110 m diameter, the inferior elements of the dioptric lens and the 

 lower catadioptric rings, constructed after this old section, emit rays that are 

 no longer in the proper direction, because the portion of light which the base 

 of the burner leaves visible becomes perceptibly nearer to the lens than in the 

 case of a three-wick burner. To lessen this defect, a graduated shape was given 

 to the burner, by placing each wick 0*002 m below the one preceding it on the 

 side of the centre. This arrangement reduces neither the regularity nor the 

 intensity of the light, and the part of that light, visible from each of the 

 lower lenticular elements, becomes somewhat increased. Moreover, these 

 lower elements have been calculated by determining for each of them a par- 

 ticular focus taken on the brightest line of the apparent part of the light, 

 instead of on the axis itself of the lamp. Similar arrangements might be 

 advantageously adopted in many cases. 



4th. The central lens and the lower rings are included in the same frame, 

 the upper rings are set in a second frame, separated from the first by a metal 

 cross-bar. In the annular lenses, this cross-bar takes the shape of the arc 



